The present invention relates to an underwater cutting granulating device used for manufacturing resin-made pellets, a knife used in this device, and an underwater cutting granulating method preferably using this knife.
This type of underwater cutting granulating device is generally provided with a die plate having a large number of die holes, a water chamber formed so as to surround the cut surface of the die plate, a knife holder housed rotatably within this water chamber, a driving means for rotating the knife holder in a fixed direction, and a plurality of knives mounted on the knife holder.
A feed water supply system for circulating cooling water is connected to the water chamber. A resin feeder such as gear pump is connected to the die, so that a fused resin from a resin kneading machine is pressurized and supplied into the water chamber.
Accordingly, the fused resin extruded from the die into the water chamber, the skin of which is cooled at the same time the fused resin is pushed into the water chamber, is finely cut by the knives, cooled and hardened within the water chamber, and granulated into pellets.
Most knives conventionally used for such an underwater cutting granulating device have, for example, a substantially triangular section as shown by the virtual line in FIG. 1 or a substantially trapezoidal section as shown in FIG. 16, wherein the rotating directional rear side (rear surface) is cut off vertically to the cut surface of the die.
When the rotating directional rear side (rear surface) of the knife is vertical to the cut surface in this way, the high-speed rotation of the knife increases the proportion of causing a turbulence in the rear thereof, which develops into a cavitation. Thus, the rotating resistance of the knife is increased by the turbulence itself or the entrainment of bubbles by the cavitation, and most of the driving force of the knife is consumed for this resistance, which contributed to a reduction in driving efficiency.
The large rotating resistance of the knife is linked to the higher flow velocity of a rotating directional water flow (different from the circulating flow of water by the feed water supply system) running along the cut surface of the die according to the rotation of the knife.
Therefore, when such a high MFR (melt flow rate) material with low viscosity that MFR exceeds 10 was used as the fused resin, the high-speed water flow trailing the knife caused a defect such as curve, deformation, crushing, hairline or the like (particularly remarkable with MFR greater than 30) in the pellet shape after cutting.
Such a pellet having a shape defect is not only deteriorated in commercial value by the visual failure, but also caught in a hopper part in extrusion or extrusion molding, which often leads to a reduction in yield or reduction in operability of the underwater cutting granulating device.
In consideration of the circumstances as described above, the present invention aims at preventing the occurrence of a shape defect even in a resin material with high MFR by reducing the rotating resistance of knife to water to suppress the turbulence and cavitation and minimizing the generation of a turbulent part trailing the knife as much as possible.
The present invention takes the following technical means in order to attain the above object.
Namely, in an underwater cutting granulating device related to the present invention, the knife of a cutter is formed so that the height dimension (h) laid along its rotating axial center is thinned, whereby the rotating resistance to water is reduced.
In this case, xe2x80x9cto thin the height dimension (h)xe2x80x9d can be realized specifically by setting the maximum distance A from the cutter surface of the knife to the inner surface (die-side surface) and the maximum thickness C of the knife to Cxe2x89xa61.2A.
Since the dimension A of this kind of knife is set to 4xcx9c6 mm in order to avoid the interference with the fused resin regularly extruded from the die holes, the maximum thickness C can be set to 4.8xcx9c7.2 mm or less.
Since the rotating resistance to water can be minimized because of the small height dimension (h) according to the present invention, the velocity of the circling directional water flow generated along the die according to the rotation of the knife can be also lowered.
Therefore, even a high MFR material can be granulated while suppressing the shape defect of pellet as much as possible, and various conventional disadvantages resulted from this shape defect can be prevented.
The minimization of the turbulence caused on the rotating directional rear side of the knife can also lead to the suppression of the cavitation, and the consumption of the cutter driving force can be consequently suppressed to enhance the driving efficiency. Further, the cut surface of the die plate can be prevented from being damaged early by the collision of a large number of bubbles generated by the cavitation with the die plate.
On the other hand, as another method for reducing the rotating resistance to water of the knife, a mountain-shaped (convex) guide surface is formed on the surface opposite to the die. In this case, a curved valley-shaped (concave) guide surface is suitably formed also on the surface facing the die so as to be conformed to the mountain-shaped guide surface.
The mountain-shaped guide surface means a surface having a part inclined away from the die in the rotating directional front part and a part inclined nearer to the die in the rotating directional rear part. Thus, the water flow generated along the mountain-shaped guide surface is drawn to the die side in the rear of the knife.
The valley-shaped guide surface means a surface having a part inclined away from the die in the rotating directional front part and a part inclined nearer to the die in the rotating directional rear part. Thus, the water flow generated along the valley-side guide surface on the inside of the knife is similarly pressed onto the die side.
Since each of the water flows laid along the mountain-shaped guide surface and the valley-shaped guide surface are headed toward the die in any case, the separation of water in the rotating directional rear side of the knife is smoothed, and the turbulence and cavitation can be suppressed.
This structure thus has the advantage that the velocity of the circling directional water flow accompanying the rotation of the knife can be lowered to prevent the shape defect in the use of high MFR materials, not to mention that the driving efficiency of the cutter can be enhanced with suppression of the consumption of the cutter driving force.
Since the down force acting on the knife is lowered when the water flow on the rotating directional rear side of the knife is thus turned to the die side, the subsidiary effect that the pressing force of the knife onto the cut surface of the die can be reduced to suppress the wear of the both and extend the lives can be provided.
The above mountain-shaped guide surface or valley-shaped guide surface has no limitation for the forming position to the knife, but it is preferably formed closer to the rotating directional rear side in order to efficiently provide the effect of heading the water flow for the die side as described above.
The mountain-shaped guide surface or valley-shaped guide surface may be formed of a bent surface as a plurality of flat surfaces are joined, but it is suitably formed into a smoothly continued curved surface in the sense of reducing the rotating resistance to water of the knife. In the formation of such a curved surface, it is more preferable to make the sectional form laid along the rotating direction of the knife into a streamline shape. The combination of flat surface with curved surface can be adapted.
Further, when both of the structure of thinning the height dimension of the knife and the structure of providing the mountain-shaped guide surface and the valley-shaped guide surface on the knife are adapted, the resulting effect can be more ensured and made more satisfactory by the synergistic effect thereof
The knife to be intended by the present invention, more specifically, has a cutter surface opposed to the cut surface of a die plate, a front surface raised from the rotating directional front edge of the cutter surface in the state inclined on the anti-cut surface side toward the rotating directional rear, a back surface extended from the rear edge of the front surface substantially backward in the rotating direction, a rear surface continued to the rear edge of the back surface, and an inner surface having an inclined part raised from the rotating directional rear edge of the cutter surface in the state inclined on the anti-cut surface side toward the rotating directional rear, and continued to the rear surface in the rotating directional rear edge thereof.
In the knife having the form described above according to the present invention, the rear surface is formed with an inclination on the cut surface side toward the rotating directional rear, and the transition angle Bxc2x0 from the front surface to the back surface and the transition angle Dxc2x0 from the back surface to the rear surface are set so as to be 150xc2x0xe2x89xa6(B+D)/2xe2x89xa6180xc2x0. Further, in the knife according to the present invention, more preferably, the maximum distance A from the cutter surface to the inner surface and the maximum thickness C of the knife are set so as to be Cxe2x89xa61.2A.
As is apparent from the FEM analytical experiment described later, the same velocity water flow part running near the die plate together with the knife is extremely shortened in the rotating directional rear of the knife even when the knife is rotated at a general rotating speed of 10xcx9c20 m/s, and the shape failure of pellets accompanying the mowing down of the used resin by the water flow part can be prevented.
In the present invention described above, the crossing line part between the rear surface and the inner surface is formed into a knife edge, whereby the turbulence of the water flow in the rotating directional rear of the knife can be more surely prevented, and the enlargement of the same velocity water flow part can be more effectively suppressed.
Further, it is recommended to provide, in the cutter surface, a part wider than the other part having cutter function in the position corresponding to the rotating directional central side part of the die plate to which the fused resin is not extruded.
In this case, since the contact area of the cutter surface to the cut surface is increased by the formation of the wider part to reduce the force per unit area acting on the cutter surface, the wear of the cutter surface of the knife and the cut surface of the die plate can be delayed as much as possible.
As long as the relation between the angle Bxc2x0 and the angle Dxc2x0 and the relation between the maximum distance A and the maximum thickness C are set as described above, the back surface of the knife can be curved on the anti-cut surface side so that no ridge line clearly appear between the front surface and the rear surface, or flatly formed so as to cross the front surface and the rear surface through ridge lines.
On the other hand, it was verified by the experiment described later that the shape failure of pellet could be solved as much as possible even in case of a high MFR resin, if L and H determined as follows were set so as to keep the relation of Lxe2x89xa64H in the operation by use of this kind of underwater cutting granulating device.
L: The rotating directional length of the same velocity water flow part formed on the rotating directional rear side of the knife and running near the die plate at substantially the same velocity as the knife.
H: The extrusion length of the fused resin extruded from the die plate during the period from the cutting by one knife to the cutting by the following knife.
If the rotating speed of the driving means of the cutter is set so that the relation of Lxe2x89xa64H is regularly established, the shape failure of pellet can be solved for the present.
However, the establishment of the relation of Lxe2x89xa64H in the state where the extruding speed of the fused resin and the rotating speed of the knife are remarkably low is not very suitable for the actual operation because the production of pellets significantly falls.
In a general operation of rotating at a high speed (10xcx9c20 m/s) by use of the above-mentioned knife of the present invention, granulation is desirably performed while holding the relation of Lxe2x89xa64H. According to the knife of the present invention, even when the driving means of the cutter is rotated at 10xcx9c20 m/s necessary for such a general operation, the shape failure of pellet can be solved by holding the relation of Lxe2x89xa64H, and occurrence of cavitation can be also effectively prevented.